1. Field of the Invention
The present invention relates generally to cementitious compositions and, more particularly, to high-strength cementitious compositions formed using bottom ash as a lightweight aggregate and pozzolan.
2. Description of Related Art
Concrete used for structural applications commonly includes a mixture of Portland cement, a fine aggregate, such as sand, and a coarse aggregate, such as broken stone or gravel. For example, a 1:2:4 concrete mixture includes one part cement, two parts sand, and four parts broken stone or gravel. Depending on the water/cement ratio, the type of Portland cement used, and the amount of time allowed for the concrete to cure, conventional concrete mixtures can provide relatively high compressive strengths, which generally are measured in terms of the concrete's three-day, seven-day, twenty-eight-day, three-month and one-year compressive strengths. For example, conventional concrete mixtures can generally be used to produce concrete having a seven-day compressive strength of between approximately 2,000 pounds per square inch (lbs/in2 or “psi”) and 4,000 psi, and a twenty-eight-day compressive strength of between approximately 3,000 psi and 6,000 psi. Most engineers require that any load-bearing concrete achieve a minimum twenty-eight-day compressive strength of 2,500 psi.
However, conventional concrete mixtures can be relatively expensive to manufacture due to the cost involved in processing the Portland cement, sand, and coarse aggregate. For example, sand used in conventional concrete mixtures must be processed so that the sand is substantially free from organic matter, vegetable loam, alkali and other deleterious substances that can adversely affect the strength of the resulting concrete. Additionally, because conventional concrete mixtures formed using cement, sand, and coarse aggregates have relatively high per unit volume weights, i.e., exceeding approximately 130 pounds per cubic foot (lbs/ft3) prior to the addition of water, conventional concrete mixtures can be difficult to package in a ready-to-use form that can be offered to both the commercial and “do-it-yourself” consumer markets.
In seeking to provide high-strength concrete mixtures that are lightweight and less expensive to manufacture and package in comparison to conventional concrete mixtures, others have proposed replacing all or a portion of the cement, sand, and/or coarse aggregates with other materials. For example, there has been considerable attention given to the use of by-products from pulverized coal combustion and refuse burning processes as a replacement for the cement, fine aggregate, and/or coarse aggregate in conventional concrete mixtures. Generally, by-products from pulverized coal combustion in thermal power stations can be categorized as fly ash, bottom ash, and slag. Fly ash comprises particles that are convected upwardly with the flue gases of a furnace and are separated therefrom using electrostatic precipitators and/or mechanical collectors. Fly ash generally includes fine particles having a relatively consistent particle size ranging from approximately 0.04 mil (1 μm) to 7.8 mil (200 μm). Bottom ash comprises heavier particles ranging in size from approximately 2 inches (5.08 cm) and less that fall to the bottom of the furnace where the particles are collected in either a dry form or in a water-filled ash pit. Slag comprises molten or partially fused particles that come into contact with the furnace wall, become chilled, and solidify. Slag is generally much denser than either fly ash or bottom ash. The use of these by-products is particularly attractive since these materials are considered to be waste products that would normally be landfilled, thus potentially providing an inexpensive and readily available lightweight aggregate and pozzolan to replace all or a portion of the cement, fine aggregate and/or course aggregate in conventional concrete mixtures.
One example of concrete mixtures that utilize a by-product of coal combustion is disclosed in U.S. Pat. Nos. 3,961,973 and 4,040,852 to Jones. The concrete mixtures disclosed in the Jones '973 and '852 patents include a mixture of fly ash and bottom ash in combination with cement and sand. These concrete mixtures have per unit volume weights of between 101.4 lbs/ft3 to 109.0 lbs/ft3, which are lower than the per unit volume weight of conventional concrete mixtures, i.e., approximately 130 lbs/ft3. However, the concrete mixtures disclosed in the '973 and '852 patents have a seven-day compressive strength of approximately 1500 psi or less and a twenty-eight-day compressive strength of approximately 2600 psi or less, both of which are considerably lower than that provided by conventional concrete mixtures. Indeed, the twenty-eight-day compressive strength of the concrete mixtures disclosed in the Jones '973 and '852 patents only narrowly exceeds the minimum twenty-eight-day compressive strength typically required by engineers for load-bearing concrete, i.e., 2,500 psi.
Another example of a concrete mixture that utilizes a by-product of coal combustion is disclosed in U.S. Pat. No. 5,849,075 to Hopkins et al. The concrete mixture of the '075 patent preferably includes cement, ground bottom ash, silica fume, coarse aggregate, and sand. The bottom ash is ground to a size in which 80% to 100% and, preferably, 85% to 90% passes a 45 μm screen, to thereby produce a highly active pozzolan. The ground bottom ash, which has a consistency and particle size similar to fly ash, preferably is mixed with silica fume and used as a partial replacement for Portland cement. Although the concrete mixture disclosed in the Hopkins '075 patent provides seven-day and twenty-eight-day compressive strengths that equal or exceed that of conventional concrete mixtures, the mixture requires the additional processing steps of grinding the bottom ash and mixing the ground bottom ash with silica fume, which increases the overall manufacturing cost of the mixture. In addition, the concrete mixture disclosed in the Hopkins '075 patent still requires both sand and a coarse aggregate, which adversely affects the per unit volume weight of the mixture and the cost for packaging the mixture.
Accordingly, there remains a need for an improved concrete mixture that has a relatively low per unit volume weight and that provides compressive strengths equal to, or exceeding, those of conventional concrete mixtures. The improved concrete mixture should be cost effective to manufacture and package and, preferably, will utilize by-products from coal combustion processes so as to provide an economically worthwhile use for these commonly landfilled by-products.